The present invention relates to a method for calendering paper, particularly with so-called glassine calendering, in the calender comprising press rolls arranged one after the other in the paper running direction. Between at least some of the rolls there will be pressing nips for passing paper therethrough. Preferably, each nip is formed by a pair of rolls, one being hard and the other elastic. Paper is advanced into the calender at an initial moisture X, which is within the range of 12%.ltoreq..times..ltoreq.25%. At least one of the nips is given a calculated line pressure which exceeds 250 kN/m.
Glassine calendering is a special application of traditional supercalendering. Paper is generally advanced into the calender at a relatively high initial moisture and the line pressures applied are substantially higher than in traditional calendering. The number of rolls and hence the total number of nips in a glassine calender exceeds that used in traditional supercalendering since, at least in some applications, such calender acts as "a drying machine" at the same time. The final moisture of the paper is typically about 6%.
In other respects the principle of glassine calendering is similar to traditional supercalendering. This means that every second roll is soft, for example a paper roll (hardness less than 95.degree. Shore D) and every second roll is an internally heated metal roll.
However, the demands set on a final product are clearly different in glassine calendering from those in supercalendering. The objective in glassine calendering is to provide a paper having a uniform thickness, high transparency and high density and having a high hold-out capacity for surface solvents.
Generally in prior known supercalenders, especially in those that are constructed from vertically superimposed rolls, the lead-in of paper into a calender or the unwinding of a roll of paper is positioned in the top section of a supercalender and a paper web is always brought out of the supercalender at the bottom section where the rewinding takes place. Thus, the paper web travels from a top nip to a bottom nip. This running mode does not produce the best possible results especially in glassine calendering since the highest line pressures are not applied to paper until the paper has almost reached its final moisture, at which time the resistance to shaping in the paper is at its peak.